FTP_User table error

Dear all,
This is my first message on the forum. I'm Diego, I work as system engineer for a small Italian company.
I'm writing this because I'm facing a strange issue with ISPCONFIG 3.
I'm trying to manage an installation made up with 2 server: a webserver and a mailserver.
I was able to create customers, sites, databases, mailboxes and everything I need, but when I tried to create an FTP User, I got a strange behaviour of the web interface. On the top of the FTP_Users page, there was a SQL error and I couldn't do anything on the page.
Taking a look to the PHP code and the MYSQL table, I found that the query in the PHP code was trying to fetch the record of the FTP_User table and "order by" a field called USERNAME. Unfortunately there wasn't any field called USERNAME in the FTP_User table!!!
I manually added the field to the table and the SQL error disappeared!! Then I tried to add an FTP user by clicking on the button, but the "FTP_USER_EDIT.php" didn't load
Do you have any advice about this?
Thank you in advance !!!
Diego

Download the ispconfig tar.gz of the version that you use on your server, there you find a file install/sql/ispconfig3.sql which contains the table definitions of an ispconfig system, search for the ftp_user table and ensure that the table on your system is exactly as the one defined there.